The present invention relates generally to chemical mechanical polishing of a substrate, and more particularly to a method and apparatus for measuring the distribution of a slurry layer delivered between a substrate and a polishing surface.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semi-conductive or insulating layers on a silicon wafer. One fabrication step involves depositing a filler layer over a patterned stop layer, and planarizing the filler layer until the stop layer is exposed. For example, a conductive filler layer may be deposited on a patterned insulating stop layer to fill the trenches or holes in the stop layer. After planarizing, the portions of the conductive layer remaining between the raised pattern of the insulating-layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate.
Chemical mechanical polishing (CMP) is one accepted method of planarizing. The method typically requires that a substrate be mounted onto a carrier or polishing head, with the exposed surface of the substrate being placed against a rotating polishing platen or belt pad. The carrier head applies a controllable pressure upon the substrate, thereby forcing its exposed surface against the polishing platen. A polishing slurry, typically including both chemically-reactive agents and a suspension of abrasive particles, is supplied to the surface of the polishing platen and serves as a polishing agent.
Because of its active role as a polishing agent, the distribution and transport of slurry on polishing platens is known to be an important parameter affecting both substrate polishing rates and polishing uniformity. Despite this, few efforts have been made to study the distribution of slurry layers as a function of CMP process parameters such as the angular velocities of polishing heads and platens, polishing head pressure, slurry flow rates, and initial slurry distribution. Recently, studies on slurry distribution in CMP have been reported in the academic literature. However, the reported measurement techniques require the use of glass substrates and cannot be used to measure slurry distribution beneath silicon substrates. Additionally, some of the reported techniques require use of permanent dyes, such as methylene blue, which tend to stain polishing platens and heads after only a single use. These techniques are therefore undesirable for use with expensive, production quality CMP machines, despite the need to use such machines in the course of developing a product line.
The invention comprises a method and apparatus for measuring the distribution of a slurry layer delivered between a platen and a substrate undergoing chemical mechanical polishing. The method involves doping a slurry solution with a light absorbing dye having an absorptivity which is sensitive to a physical parameter. In one implementation the absorptivity of the dye is sensitive to the acidity of the slurry. In another implementation, it is sensitive to temperature, and in a third implementation it is sensitive to pressure. The doped slurry is delivered to the surface of the platen and forms a layer between the platen and a substrate undergoing polishing.
A light source emits a beam of light which is transmitted through the slurry layer, reflected off of the substrate, and detected by a photodetector. The light source may emit the light at a predetermined intensity, or at a variable intensity which is measured. The intensity of the transmitted light is measured by the photodetector and the transmittance through the doped slurry layer is thereby determined. The thickness of the slurry layer may be determined from the transmittance, the absorptivity of the doped slurry solution, and a physical model of the slurry distribution. In one implementation, variations in the slurry layer thickness are small, and the thickness of the slurry layer is determined from the transmittance and the absorptivity.
The light source and substrate are in relative motion. This allows the thickness of the slurry layer to be determined as a function of both the radial and azimuthal position of the light source with respect to a fixed position on the substrate, taken to be its center. In one implementation, both the light source and the substrate are rotated around a central axis. In that implementation the slurry layer has no azimuthal angular dependency, and the slurry layer distribution is determined as a function of radial distance from the center of the substrate.